Support for noncopyable `Service`

[RandomHashTags]

It would be nice if there was a noncopyable Service protocol as I am currently required to compromise the data structure in one of my projects heavily utilizing noncopyable types. Or at the very least make run() mutating.

[FranzBusch]

At this point we cannot adopt ~Copyable for Services since we need move the service into a child task. Until we get call-once closures that's not going to work nicely. Let's keep this issue open for tracking purposes until the language provides such call-once closures.